backend-vnc: pass vnc_backend to vnc_head_create()

Pass the VNC backend to vnc_head_create().

That way the already known backend will not have to be found from the
compositor. This is trivial now, but in the multi-backend case would
entail iterating over all backends to find the correct one.

Also remove the now unused to_vnc_backend() helper.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>